1. Field of the Invention
This invention relates to corrosion inhibition and, in particular, to corrosion inhibition by potable waters such as municipal drinking waters and the like.
2. Brief Description of the Prior Art
Treatment of water with soluble salts such as alkali metal hexametaphosphates has been practiced for many years. The hexametaphosphate salts were initially employed in threshold treatment of waters such as boiler feed water and the like since it was recognized that the hexametaphosphate was a strong complexing agent for iron and manganese, preventing their precipitation and resultant scale formation on the equipment of steam power plants.
An outgrowth of the threshold treatment of water was the discovery that the hexametaphosphate salts could also be employed to inhibit or prevent corrosion of iron and mild steel surfaces. The exact mechanism of this corrosion prevention is not entirely understood although it is generally believed that the hexametaphosphate forms a protective, passivating film on the metal surfaces. It has long been recognized that the hexametaphosphate must be employed at concentrations of at least about 50 or 60 ppm to achieve passivating of fresh metal surfaces, following which the concentration can be reduced to about 20-30 ppm to maintain the film. Attempts to achieve corrosion prevention with lower concentrations of the hexametaphosphate have not been successful in reducing the overall corrosion rate, although some investigators have reported that hexametaphosphate salts have been observed to reduce the pitting nature of the corrosion when employed in low concentration.
It is desirable to minimize the quantities of phosphate additives in water to lessen the risk of promoting growth of algae and aquatic plants. It is also desirable to minimize the amounts of complex phosphates, such as hexametaphosphate in water to avoid undesirable coagulation problems.